1. Field of the Invention
This invention relates generally to fluid flow measuring devices and more particularly to ultrasonic devices in which a Karman vortex street is induced in a flowing fluid by placing a vortex-generating obstruction in the flowing fluid and processing the resulting vortices so as to amplitude-modulate an electrical carrier signal.
2. Description of the Prior Art
Flow meters are known in which vortices are induced in a fluid and then counted by some means to provide an indication of the velocity of fluid flow. In some representative types, incorporated signal amplifiers are operated in a saturated mode and switched from positive to negative saturation to generate rectangular waves at the vortex occurring frequency. Aside from the harmonic noise generated by such a device, switching has been found to be erratic as a result of low input signal amplitudes and to exhibit a dead-band between positive and negative going input signals that contributes to further erratic behavior of such a system. In many instances, a flow rate of from zero to some upper limit would be characterized by an output signal over a range of currents different from zero to some upper limit. For example, in one such system, a range of output currents of from 4 ma to 20 ma corresponding to a flow rate range of from zero to about 20 feet/second is reported with no mention of the linearity of the system. Such an offset of output signal from input phenomenon can cause calibration difficulties along with the readout problems associated with a non-linear system.
Many different circuits and configurations of elements have been employed in the prior art in attempts to solve the problems presented. Most have either presented new problems or only partially solved those existing problems, or both.
Among circuit designs in use, one applies fixed amplification to the received ultrasonic carrier signal, while another applies automatic gain control to the amplification of the received ultrasonic carrier signal in an effort to stabilize the signal amplitude at the amplifier output. In the former system, when carrier signal amplitude is low, the modulating signal is also low in amplitude resulting in improper functioning under most required operating conditions. In the latter system, poor operation results from the presence of inherently high internally generated noise in most high gain integrated circuit amplifiers under conditions that the automatic gain control is not activated by a relatively high carrier signal amplitude. Thus, as carrier strength decreases, the gain of the amplifier increases and amplifies its own inherent, internally generated noise until at some gain level the amplitude of the thus-amplified noise signals is as high as the normal minimum modulating signal amplitude and the noise is thus detected as flow data. Many of these devices have thus met special needs as presented by specific problems and have therefore served narrow purposes. These prior art devices, among other disadvantages, have been unreliable and unpredictable in operation under continued use and have been expensive and complicated to manufacture. Some of these prior art devices have been described in the following listed patents which were brought to the attention of the applicant through a novelty search conducted in the United States Patent and Trademark Office:
1. U.S. Pat. No. 3,413,564--Seifried PA0 2. U.S. Pat. No. 3,948,098--Richardson et al PA0 3. U.S. Pat. No. 3,854,334--Herzl
Issued: Nov. 26, 1968
"Combined A.G.C. and Signal Indicator" Shows a combined A.G.C. and failure indicator circuit.
Issued: Apr. 6, 1976
"Vortex Flow Meter Transmitter Including Piezo-Electric Sensor" Shows a fluid flow instrument using vortices generated by a piezo-electric crystal to measure fluid flow by rate of pulses generated by the crystal.
Issued: Dec. 17, 1974
"Signal Recovery System for Vortex Type Flow Meter"
Shows a fluid flow meter of the vortex type using a sensor (disclosed as such only generally) and which includes an "Electronic Flywheel" in measuring the pulsation rate signal output of the sensor.
It would thus be a great advantage to the art to provide a superior method of amplification of amplitude-modulated, ultrasonic frequency electronic signals such as are employed in fluid flow measuring devices.
It would be a further great advantage to the art to provide superior demodulation capabilities to perform signal detection in fluid flow measurement systems.
It would also be of advantage to the art to provide a detectable signal indicating a fault in the event that the acoustic path is broken during operation of the flow measuring device.
An additional important advantage would be the provision of the above listed advantages in a manner such that existing systems might be modified so as to obtain the advantages sought by the present invention.